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Abstract:

A multilayer light modulator includes, a light modulating stack operable
to transform an electrical control signal into a modulated optical
signal. The light modulating stack comprises one or more optically
reflecting layers, optically transmitting layers, and optically variable
layers. The optically variable layer comprises a plurality of
electrophoretic particles supported in a fluid. The multilayer light
modulator also includes a bias generator coupled to the optically
variable layers. The bias generator is responsive to the electrical
control signal, wherein the bias generator creates a bias that changes
the reflectance of the light modulating stack by causing the
electrophoretic particles to move within the fluid.

Claims:

1. A multilayer light modulator comprising:a light modulating stack
operable to transform an electrical control signal into a modulated
optical signal, the light modulating stack comprising:one or more
optically reflecting layers;one or more optically transmitting layers;one
or more optically variable layers, the optically variable layers
comprising a plurality of electrophoretic particles supported in a
fluid;a bias generator coupled to the optically variable layers and
responsive to the electrical control signal, wherein the bias generator
creates a bias that changes the reflectance of the light modulating stack
by causing the electrophoretic particles to move within the fluid.

2. The multilayer light modulator of claim 1, wherein the fluid comprises
a material selected from the group consisting of water, hexane, decane,
isoparaffinic fluid, and methanol.

4. A device comprising:a first layer having a first index of refraction;a
second layer having a second index of refraction;a fluid having a third
index of refraction enclosed between the first layer and the second
layer;a plurality of particles having a fourth index of refraction
suspended in the fluid; anda variable voltage source coupled to the first
and second layer and operable to cause the plurality of particles to move
within the fluid.

5. The device of claim 4, wherein the plurality of particles are
electrically charged.

6. The device of claim 4, wherein the fluid is of a first viscosity, the
first viscosity such that the particles move within the fluid when an
electrical force is applied to the particles and are suspended when no
electrical force is applied to the particles.

7. The device of claim 4, wherein the particles diffuse light.

8. The device of claim 4, wherein the first refractive index is different
than the second refractive index.

9. The device of claim 4, wherein the fluid is substantially optically
transparent.

10. The device of claim 4, further comprising at least one additional
layer having at least one additional index of refraction.

11. The device of claim 4, wherein the first, second, third, and fourth
indices of refraction are selected to maximize a change in reflectance of
the device upon the particles moving from a first position to a second
position.

12. The device of claim 4, wherein the first and second layers comprise a
material selected from the group consisting of silicon, germanium,
silicon nitride, rutile, and diamond.

13. The device of claim 4, further comprising a layer of capsules arranged
between the first layer and the second layer, the capsules operable to
contain the fluid between the first layer and the second layer.

14. The device of claim 4, wherein the plurality of particles comprises a
plurality of metallic flakes.

15. A method comprising:receiving an electrical control signal;adjusting
the reflectance of a light modulating stack comprising a plurality of
layers by changing the relative location of a plurality of
electrophoretic particles with respect to the remainder of the light
modulating stack;receiving light waves from a light source; andconverting
the electrical control signal into a modulated optical signal that
selectively switches between reflecting or absorbing at least a portion
of the light waves.

16. The method of claim 15, wherein adjusting the reflectance of the light
modulating stack comprises adjusting a bias applied to at least one layer
of the light modulating stack.

17. The method of claim 15, wherein upon the reflectance being adjusted,
the bias is no longer applied to the light modulating stack.

18. The method of claim 15, wherein adjusting the reflectance of the light
modulating stack comprises creating a layer of electrophoretic particles
against an inner surface of one layer of the plurality of layers of the
light modulating stack.

19. The method of claim 18, further comprising maintaining the layer of
electrophoretic particles until the electrical control signal indicates a
change in the reflectance of the light modulating stack.

20. The method of claim 15, wherein receiving an electrical control signal
comprises receiving an electrical control signal-indicating the
reflectance of the light modulating stack is to produce a modulating
optical signal of a predetermined pattern.

Description:

TECHNICAL FIELD OF THE INVENTION

[0001]This invention relates in general to light modulators and, more
particularly, to a device and method for implementing a multilayer light
modulator.

BACKGROUND

[0002]It is generally useful to provide an electronic means to modulate
the intensity of optical radiation. Applications of such modulators
include spatial light modulators, optical displays, and the like. Example
light modulators include liquid crystal, electrochromic,
electromechanical, Bragg, and others. Liquid crystal modulators rely on
rotating the plane of polarization to modulate light. More specifically,
a fixed polarizer is used to polarize the incident light so that the
rotation effect may be controlled and employed to form a light modulating
element. An electric field can be used to alter the degree of rotation
and thereby alter the effective transmission or reflection from this type
of modulator. Electrochromic modulators inject ions into (or out of) a
material such that the material changes from being optically transparent
to optically absorbing. A back reflector (e.g., a metal mirror) is placed
behind this material so that light may either be reflected by this mirror
back to the observer, or absorbed by the electrochromic layer. The mirror
is perforated so that ions may pass through the mirror.

SUMMARY

[0003]In accordance with a particular embodiment, a multilayer light
modulator includes a light modulating stack operable to transform an
electrical control signal into a modulated optical signal. The light
modulating stack comprises one or more optically reflecting layers,
optically transmitting layers, and optically variable layers. The
optically variable layers comprise a plurality of electrophoretic
particles supported in a fluid. The multilayer light modulator also
includes a bias generator coupled to the optically variable layers. The
bias generator is responsive to the electrical control signal, wherein
the bias generator creates a bias that changes the reflectance of the
light modulating stack by causing the electrophoretic particles to move
within the fluid.

[0004]Technical advantages of particular embodiments include the ability
to provide a light modulator suitable for use over a specified range of
optical frequencies such that light may be reflected from an optical
surface with controllable intensity.

[0005]Other technical advantages may be readily apparent to one skilled in
the art from the following figures, descriptions, and claims. Moreover,
while specific advantages have been enumerated above, various embodiments
may include all, some, or none of the enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]For a more complete understanding of particular embodiments and
their advantages, reference is now made to the following description
taken in conjunction with the accompanying drawings, in which:

[0007]FIG. 1 depicts a multilayer optical interference filter;

[0008]FIGS. 2A and 2B depict a multilayer light modulator in two different
states, in accordance with particular embodiments;

[0009]FIG. 2c depicts a multilayer light modulator comprising a layer of
microencapsulated electrophoretic material, in accordance with particular
embodiments; and

[0010]FIG. 3 depicts a method for implementing a multilayer light
modulator, in accordance with particular embodiments.

DETAILED DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 depicts a multilayer optical interference filter. Multilayer
optical interference filter 100 is formed by combining several layers of
generally planar, optically transparent materials and arranging them in a
vertical stack. The combination of layers may cause light 110 incident
upon filter 100 to undergo multiple internal reflections (120a-120f). As
will be discussed in greater detail below, the characteristics of
reflections 120 may depend on the materials used in layers 101, 103, 105,
107, and 109 and the differences between the materials of respective
adjacent layers.

[0012]Filter 100 may comprise obverse side 160 and reverse side 150.
Obverse side 160 of stack 100 represents the viewing surface to which
light 110 may be incident. This light may come from a light source that
may be natural or artificial in nature. Light reflected from stack 100 is
composed of light rays or light waves that reflect from the top viewing
surface 160 (reflected light 120a) and light that reflects from each
optical interface between all of the layers of stack 100 (reflected light
120b-120f). The collection of reflected light 120 from stack 100 may be
redirected by the reflection process to be viewed by a user or detected
by an optical sensor.

[0013]In a light modulator, electrophoretic particles may be used to
change the reflection and refraction at the interfaces between one or
more respective adjacent layers of filter 100. By using movable
electrophoretic particles suspended in fluid for one or more of the
layers of filter 100, it may be possible to change the optical boundary
conditions between one or more layers. Thus, light 110 incident upon
filter 100 may be either strongly reflected or strongly absorbed due to a
change in the position of the particles. In other words, the optical
interference of the combined filter layers and electrophoretic material
may change based on the position of the particles.

[0014]As mentioned above, light 110 may be reflected and refracted at the
boundary between two optically dissimilar materials. The optical
reflectance of the interface between two layers may be calculated using
an equation, [(n1-n2)/(n1+n2)] 2, where n1 and n2 are the indexes of
refraction for two adjacent layers (e.g., layers 101 and 103). In
general, a high reflectance at an interface between two materials
requires that the index of refraction of the two materials be as
different as possible. For example, the reflectance of silicon
(index=3.7) in air is about [(3.7-1)/(3.7+1)] 2=33%. However, the
reflectance of polyester film (index-1.5) in air is only about
[(1.5-1)/(1.5+1)] 2=4%. Thus, if it is desired to provide high
reflectance using a minimum number of layers of material, it is best to
use materials that have highly dissimilar indexes of refraction.

[0015]By switching the location of two adjacent layers, such as layers 105
and 107, without changing the properties of the respective layers, the
average reflectance of filter 100 may be significantly altered. For
example, if layers 101, 105, and 109 had an index of 1.4 and layers 103
and 107 had an index of 3.7, then each interface between the layers may
comprise materials having relatively dissimilar indexes of refraction.
Then, if layers 105 and 107 were switched, so that layers 103 and 107 are
adjacent and layers 105 and 109 are adjacent, at least two of the
interfaces between the layers may comprise materials having relatively
similar indexes of refraction (layers 103 and 107 both have an index of
3.7, and layers 105 and 109 both have an index of 1.4). In this example,
the switch may change the average reflectance of light 110 (over the 1 to
12 micrometers spectral band) by 34.6%. Thus, changing the order of light
absorbing and light transmitting layers can significantly alter the
optical reflectance of an interference stack.

[0016]FIGS. 2A and 2B depict a multilayer light modulator in two different
states, in accordance with particular embodiments. In FIG. 2A, the
majority of the electrophoretic particles 240 create a thin film of
optically absorbing material just under layer 203; and in FIG. 2B the
majority of the electrophoretic particles 240 create a similarly thin
film of optically absorbing material on top of layer 209. Thus, in
essence, FIG. 2 illustrate the effective re-ordering of the layers in the
stack of multilayer light modulator 200 by moving electrophoretic
particles 240 within an optically transparent fluid 230. In other words,
fluid 230 represents one layer, and particles 240 represent another
layer. Thus, moving particles 240 between the two steady states shown in
FIG. 2 essentially switches the position of two layers of the stack,
thereby creating multilayer light modulator 200.

[0017]A light modulator for a desired spectral band may be designed by
taking into account the variations in optical properties of the material
used in the stack. For example, the plastic or glass materials that form
the layers of the stack may be selected, in part, based on the optical
refractive index of fluid 203. In particular embodiments, the layers of
the stack may comprise materials that introduce large discontinuities in
the refractive index between layers. This may improve the modulation
efficiency of a modulator over specific ranges of optical frequencies.
Thus, depending on the embodiment, multilayer light modulator 200 may be
designed to provide a degree of optical modulation across a relatively
narrow or relatively broad range of optical frequencies.

[0018]Light modulator 200, as depicted in FIG. 2, comprises stack 200 made
up of layers 201, 203, and 209 with particles 240 and fluid 230 acting as
two additional layers. Light modulator 200 further comprises voltage
source 250 for providing the necessary bias to move particles between the
steady states depicted in FIGS. 2A and 2B. Voltage source 250 may be
responsive to control signal 260.

[0019]As indicated above, the composition (e.g., thickness, dielectric
constant, index of refraction, etc.) and arrangement of layers 201, 203,
and 209 may affect the center frequency and bandwidth of the wavelengths
over which optical modulation occurs. In certain embodiments, layer 201
may be generally transparent at the optical frequencies over which light
modulator 200 is to operate.

[0020]In particular embodiments, fluid 230 may be contained by layers 203
and 209. In certain instances, at least one of layers 203 or 209 may
comprise a material of high refractive index such as silicon, germanium,
silicon nitride, rutile, diamond, or the like. Depending on the design
requirements for multilayer light modulator 200, it may be desirable to
select a material having low optical loss and a high refractive index.
For example, a light modulator designed for the 2-12 micron region of the
infrared spectrum might use silicon or germanium. Diamond or rutile may
be appropriate for use in light modulators designed for the visible
spectrum.

[0021]Once the materials of layers 201, 203, and 209 are known, the
thickness of each layer may be determined. In some embodiments it may be
desirable to use a thickness that maximizes the change in the reflectance
of light incident upon the light modulator when electrophoretic particles
240 move from one encapsulating surface to the other. In determining the
thickness, it may also be desirable to take into account characteristics
of particles 240.

[0022]In particular embodiments, layers 203 and 209 may be arranged so as
to form a parallel plate capacitor with fluid 230 and the suspended
electrophoretic particles 240 filling the space between the plates of the
parallel plate capacitor. In particular embodiments, layers 203 and 209
may, in essence, create the cavity in which fluid 230 is contained.

[0023]Electrophoresis typically requires a relatively small amount of
electrical current. In some embodiments, the electrical conductivity of
layers 203 and 209 may be relatively low. Electrical resistances of
several tens-of-thousands of ohms per square of material may be
sufficient to establish a uniform electrical field between layers 203 and
209. The uniform electrical field may provide uniform electrical forces
that may help to uniformly pack electrophoretic particles 240 against the
respective electrode.

[0024]In certain embodiments, layer 209 may comprise an optically
transparent material having a thickness and optical index of refraction
that will reflect light normally incident upon the outer surface of layer
201 with relatively high efficiency when the electrophoretic particles
240 are moved away from layer 209 and with relatively low efficiency when
particles 240 are moved towards layer 209.

[0025]By including layers 201, 203, and 209 with the electrophoretic
modulator (fluid 230 and particles 240) multilayer light modulator 200
may be able to provide the ability to modulate certain wavelengths of
light with a higher contrast than a device that uses only an
electrophoretic modulator. The additional layers may also allow
multilayer light modulator 200 to reflect light in a specular
(mirror-like) manner. For example, multilayer light modulator 200 may be
able to provide a relatively high specular reflectance in one state, and
relatively high absorbance in another state. At least part of the reason
the additional layers 201, 203, and 209 allow for improved specular
reflectance is that they may help compensate for the fact that the use of
particles 240 may, by their very nature, tend to randomly diffuse light
thereby scattering it in undesired directions.

[0026]As mentioned above, fluid 230 fills the void between layers 203 and
209 and provides the medium in which particles 240 are suspended. It may
be desirable for fluid 230 to have a viscosity that allows particles 240
to move freely within fluid 230 when biased yet remain still when the
bias is no longer being applied. This allows for relatively low power
consumption because layers 203 and 209 need only be biased long enough to
move particles 240 to the desired surface and fluid 240 prevents the
particles from drifting away. In certain embodiments, fluid 230 may
comprise a fluid selected to be of the lowest possible refractive index.
For example fluid 230 may be water, hexane, decane, methanol, or other
organic fluids. In some embodiments, fluid 230 may be an isoparaffinic
fluid which may have relatively low electrical conductivity. If the
particles selected are able to move in air, then air may be an ideal
carrier fluid since it has the lowest index of refraction. As a practical
matter, fluid 230 may at least have a relatively small optical absorption
(e.g., k=0.1) due to the fact that some of the individual particles may
not be electrified or may otherwise remain in suspension contributing to
a background level of absorption.

[0027]As alluded to above, the material used for particles 240 may have an
impact on the overall performance of light modulator 200. The
characteristics of particles 240 may vary greatly depending on the
intended application and/or desired results. Some of the characteristics
that may change include the charge of the particles, the size of the
particles, the color of the particles, the material of the particles, the
shape of the particles and any other characteristics that may affect one
or more characteristics of the stack. With respect to the size of
particles 240, it is generally understood that the modulation bandwidth
of a light modulator based on electrophoresis is generally inversely
proportional to the size of the individual particles. Thus, it may be
desirable to use relatively small particles. These small particles tend
to scatter incident light in a non-specular manner making it difficult to
obtain mirror-like reflections. However, these same small particles may
be well suited to providing diffuse scattering and/or optical absorption.
Thus, when designing a multilayer light modulator it may be desirable to
use electrophoretic particles to remove optical energy from the system
rather than to specularly reflect optical energy within the system.

[0028]In some embodiments, particles 240 may comprise particles of
silicon, appropriately doped to induce optical absorption and electrified
to act as electrophoretic particles. Silicon particles 240 may be able to
lie down to produce a thin film with an effective index of refraction of
about 3.68 which may approximate a solid film of optically absorbing
silicon. If the packing of particles 240 contains voids (e.g., in some
scenarios particles 240 may only be 50% compacted), then the effective
dielectric constant of the packed film may be a linear combination of the
dielectric constant for particles 240 and suspending fluid 230.
Optimizing light modulator 200 may require using particles 240 and a
suspending fluid 230 that result in maximal and minimal reflectance in
the two states depicted in FIG. 2.

[0029]Carbon may be another material of choice for particles 240. Carbon
is easily charged electrically and is a well known electrophoretic
material. Carbon also has a strong optical absorption characteristic over
a very wide spectral band so that electrophoretic carbon may be used over
a range of wavelengths spanning the visible spectrum through radio
frequencies. For example, graphite carbon displays an index of refraction
(n) between 2.0 to 2.4 in the wavelength range of 0.5 to 12 micrometers.
This structure of carbon has an optical absorption constant (k) of 1.3
and 8.75 in the same spectral range. Other particles that may make for
useful electrophoretic materials include rutile particles, metal spheres,
metallized dielectrics, planar metal sheets, metallic flakes, and
particles of high dielectric constant that may have essentially no
optical absorption, and the like.

[0030]Because electrophoretic particles 240 may be contained between
layers 203 and 209, it may be that particles 240 may only be able to
affect the optical properties at the interfaces with layers 203 and 209.
In other words, particles 240 may be constrained by the bounds of their
encapsulating materials so that only the inner surfaces of the
encapsulating layers 203 and 209 can be closely approached by these
particles. Accordingly, particles 240 may be too far removed from layer
201 to provide any tangible effect on the optical properties of layer
201.

[0031]In particular embodiments, electrical control signals may be
converted into a modulated optical signal by adjusting the bias applied
by variable voltage source 250. Because electrophoretic particles may be
made to move under the influence of an electric field, the motion of
particles 240 may be accomplished by making layers 203 and 209
electrically conducting. Then, a bias applied between the two layers may
cause the electrophoretic particles 240 to move toward or away from, for
example, layer 203 of multilayer light modulator 200. As indicated above,
this motion of particles 240 may cause a change in reflectance of the
obverse side of multilayer light modulator 200 so that light may be
reflected from or absorbed by multilayer light modulator 200 based on the
electrical control signal 260.

[0032]The bias between layers 203 and 209 may then be adjusted via voltage
source 250 in response to changes in control signal 260. As discussed
above, switching the bias applied to layers 203 and 209 may cause
particles 240 to move towards one or the other of the layers. In
particular embodiments, the bias may be supplied by a battery. The rate
at which particles 240 may switch between the states depicted in FIG. 2A
and FIG. 2B may exceed typical video rates. For example, in some
scenarios light modulator 200 may be capable of switching between states
in excess of 100 switches per second.

[0033]Besides the relative bias of layers 203 and 209, the direction in
which particles 240 move may also depend on the charge of the particles
themselves. For example, assume that at a particular moment in time layer
203 is biased at a higher potential than layer 209, then if particles 240
are positively charged they would move towards layer 209 and if particles
240 are negatively charged then they would move towards layer 203.

[0034]Because multilayer light modulator 200 relies on optical
interference for its modulation, it may be desirable for the layers to be
relatively planar and not include sources of scattering (as would happen
if amorphous or disordered materials were used). Also, because layers 203
and 209 are a pair of electrodes that generate the electric fields used
to move the electrophoretic particles, it may be desirable to take into
account any changes in the optical properties of the otherwise
non-conducting layers. Layers 201, 203, and 209 of multilayer light
modulator 200 may be arranged so as to take advantage of the ability of
electrophoretic particles to scatter and/or absorb optical energy. For
example, in certain embodiments, one of layers 203 or 209 may have a
relatively low refractive index and the other layer 209 or 203 may have a
relatively high refractive index. Thus, moving electrophoretic particles
240 may alter the reflectance at the boundary between a high index and
low index material. In particular embodiments, it may be desirable for
electrophoretic particles 240 to be encapsulated by a material that has a
large difference in refractive index or optical absorption (or both)
between particles 240 and that of layers 203 and 209 and the suspending
fluid 230.

[0035]While the stack of the depicted multilayer light modulator 200
includes three layers (201, 203 and 209) and a single electrophoretic
modulator (fluid 230 and particles 240), other embodiments may include
more or less optical layers as well as additional electrophoretic
modulators. For example, in FIG. 2c the gap between layers 203 and 209 is
filled with a layer of microencapsulated electrophoretic material. The
microencapsulated electrophoretic material may comprise a plurality of
individual capsules 270c filled with fluid 230c and particles 240c. In
using capsules 270, it may be desirable to account for the optical
properties and index of refraction of the upper and lower surfaces of the
capsule. In other words, using capsules 270 creates two additional layers
as compared to multilayer light modulators 200a or 200b. The use of
individual capsules 270 may simplify the handling and fabrication of
multilayer light modulator 200c.

[0036]In some embodiments, a specific coating (e.g., transparent oxides,
oxynitrides, carbon nanotubes, etc.) may be introduced to provide
electrical conductivity rather than incorporate electrical conductance
into an otherwise non-conducting layer. Furthermore, the layers may
include any variations in thickness that may provide useful values of
modulation contrast over specific optical frequencies.

[0037]FIG. 3 depicts a method for implementing a multilayer light
modulator, in accordance with particular embodiments. For purposes of the
depicted method it may be assumed that the multilayer light modulator
comprises a stack similar to the stack of FIG. 2A.

[0038]The method begins at step 310 with the stack receiving light waves
from a light source. The light source may comprise both natural (e.g.,
the sun) and artificial (e.g., light emitting diode) sources of light.
The spectrum of the light waves received by the light modulator may vary
depending on the source from which they are received. For example, light
waves from the sun may comprise a broad spectral range, whereas light
waves from an LED may be more narrowly directed to a more specific
spectral range (e.g., the range of spectrum associated with a particular
shade of blue).

[0039]At step 320 the stack receives an electrical control signal. The
electrical control signal and the light waves (step 310) may be received
concurrently. More specifically, in some embodiments, the stack may be
exposed such that whenever the sun is out, the stack receives sunlight.
Accordingly, when the electrical control signal is being received, so to
are the light waves from the sun. As discussed above, the electrical
control signal may comprise information that may be used to adjust the
bias supplied by a voltage source. By controlling the bias, the
electrical control signal may arrange the stack to reflect or absorb all,
some or none of the light waves from the light source.

[0040]At step 330 the stack adjusts its reflectance. More specifically,
the voltage source adjusts the bias applied to one or two of the layers
of the stack. The change in bias may cause electrophoretic particles,
suspended in a fluid encased between two layers of the stack, to move
away from their current position. For example, if the electrophoretic
particles are currently spread in a thin film along a bottom surface of
the upper layer of the two layers encasing the fluid, then adjusting the
bias may cause the particles to move towards the top surface of the lower
layer.

[0041]At step 340 the stack converts the electrical control signal into a
modulated optical signal. More specifically, variations or changes in the
electrical control signal may adjust the reflectance of the stack. The
changes in reflectance affect the light waves that are reflected by the
stack. These changes to the reflected light waves make up the variations
in the optical signal. It should be noted that the reflected light waves
may vary in spectrum from the light waves that are received at step 310.
The range of spectrum of the light waves which may be modulated may
depend on the material and/or arrangement of the layers of the stack. In
other words, the stack may be designed to operate within a relatively
wide or narrow range of spectrum.

[0042]Modifications, additions, or omissions may be made to the method
depicted in FIG. 3. For example, the flowchart may include more, fewer,
or other steps. Additionally, steps may be performed in any suitable
order and by any suitable component.

[0043]Although particular embodiments have been described a myriad of
changes, variations, alterations, transformations, modifications and
alternate embodiments may be suggested to one skilled in the art, and it
is intended that the present invention encompass such changes,
variations, alterations, transformations, and modifications as falling
within the scope of the appended claims.

Patent applications by Gary A. Frazier, Garland, TX US

Patent applications by Raytheon Company

Patent applications in class Changing position or orientation of suspended particles

Patent applications in all subclasses Changing position or orientation of suspended particles